The present disclosure relates to packaging and making interconnections of transducer arrays, particularly to packaging and interconnections of capacitive micromachined ultrasonic transducer (cMUT) arrays.
The packaging or making interconnections of a transducer array on a proper substrate is a key for any application using a transducer array, especially for a transducer array with large number of elements. One of exemplary applications is ultrasound imaging which uses an ultrasound transducer arrays. The proper packaging and interconnections of the ultrasound transducer array is very important to achieve desired performance with lower cost.
Capacitive micromachined ultrasonic transducers (cMUTs) are electrostatic actuators/transducers, which are widely used in various applications. Ultrasonic transducers can operate in a variety of media including liquids, solids and gas. Ultrasonic transducers are commonly used for medical imaging for diagnostics and therapy, biochemical imaging, non-destructive evaluation of materials, sonar, communication, proximity sensors, gas flow measurements, in-situ process monitoring, acoustic microscopy, underwater sensing and imaging, and numerous other practical applications. A typical structure of a cMUT is a parallel plate capacitor with a rigid bottom electrode and a movable top electrode residing on or within a flexible membrane, which is used to transmit/accurate (TX) or receive/detect (RX) an acoustic wave in an adjacent medium. A direct current (DC) bias voltage may be applied between the electrodes to deflect the membrane to an optimum position for cMUT operation, usually with the goal of maximizing sensitivity and bandwidth. During transmission an alternating current (AC) signal is applied to the transducer. The alternating electrostatic force between the top electrode and the bottom electrode actuates the membrane in order to deliver acoustic energy into the medium surrounding the cMUT. During reception an impinging acoustic wave causes the membrane to vibrate, thus altering the capacitance between the two electrodes.
For actual application, a cMUT array needs to be packaged on a print circuit board (PCB) or a substrate with desired circuits (electrical wires, pads or other electrical components) to interface with outside. Usually a cMUT array is diced from its fabrication process wafer (substrate) and attached on a PCB. A wire-bonding, flip-chip bonding or other standard IC packaging methods is then used to electrically connect the cMUT array with the wires, pads or the circuits on a PCB. In the existing packaging techniques, a PCB or a packaging substrate of the cMUT transducer is manufactured separately and the finished packaging substrate (e.g., a PCB) and the finished cMUT are attached together by a glue layer such as epoxy.
Many techniques have been developed in semiconductor industry to make interconnections between a device chip and a PCB. Two typical methods are wire-bonding and flip-chip bonding. Using the standard semiconductor packaging methods, the conventional cMUT packaging methods first make a PCB or a packaging substrate with a desired design, then assemble all components (e.g., cMUT transducers, IC chips, inductors, switches) on the PCB board, and finally make the connections between the components using either wire bonding, soldering or flip-chip bonding method. The electrical connection is done either by single wire connection each time or one chip/component each time, and is thus time-consuming for a system with a large number of components and an array with a large number of elements. In addition, the device insulation using thin film dielectric material coating is difficult to achieve using these conventional packaging methods. Furthermore, both flip-chip bonding and wire bonding need a large footprint which occupy a lot of area. Therefore, a better packaging and connection method is needed.